Automatic volume control system



May 1 7 1938.,

P. F. G. HOLST AUTOMATIC VOLUME'CONTROL SYSTEM Filed June 29, 1935 6Sheets-Sheet l GRID 5/45 0) NEXT AEAMPL.

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70 05C. ANODE 70AM HEATER-S INVENTOR POUL F. 6. HOLST BY MW ATTORNEY May17, 1938. F, G HOLST 2,117,664

AUTOMATIC VOLUME CONTROL SYSTEM Filed June 29, 1955 6 SheetsSheet 2 7F/XED 8/145 0/005 ACT/V5 CONTROLZED 5/6/1641 6/7/0 8/45 0 s/a/v/uSTRENGTH AMPL.

INVENTOR POUL F G. HOLST ATTORNEY 6 Sheets-She et 3 May 17, 1938. P. F.G. HOLST AUTOMATIC VOLUME CONTROL SYSTEM Filed June 29, 1935 May 17,193.8.

b. F, e. HQLST 2,117,664

AUTOMATIC VOLUME CONTROL SYSTEM 6 Sheets-Sheet 5 Filed June 29, 1935 AllIII" POUL F.6. HOLST ATTORNEY P. F. G. HOLST AUTOMATIC VOLUME CONTROLSYSTEM Filed June 29, 1955 6 Sheets-Sheet 6 INVENTOR POUL F. 6. HOL'STBY 4mm ATTORNEY May 17, 1938.

Patented May 17, 1938 UNITED STATES ATENT OFFIQE Poul F. G. Holst,Audubon, N. J., assignor to Radio Corporation of America, a corporationof Delaware Application June 29, 1935, Serial No. 29,014

16 Claims.

My present invention relates to control devices for high frequencysignaling systems, and more particularly to improved automatic gaincontrol mechanisms for radio signaling systems.

In the co-pending application of W. L. Carlson, application Serial No.29,003, filed June 29, 1935 there has been disclosed an automatic volumecontrol arrangement for a radio receiver wherein the anode of the gaincontrol diode is normally maintained positive with respect to thecathodes of the tubes whose gain is under control; and during suchnormal condition of the receiver the initial negative bias of thecontrol grids of the controlled amplifiers is provided by an auxiliarydiode circuit. In such a prior arrangement delayed AVC action issecured; the initial grid bias of the controlled stages being replacedby the gain control bias upon the impression on the receiver of a signalhaving an amplitude above a predetermined intensity level.

One of the main objects of the present invention is to provide improveddelayed AVC systems for radio receivers, which systems gen erally followthe mode of operation disclosed in the aforesaid Carlson application;however, the present arrangements differing from the latter priorarrangement in that the fixed grid bias diode circuit and the automaticgain control bias diode circuit are energized from a common directcurrent Voltage source.

Another important object of the present invention may be said to embodythe provision of a highly commercial type of automatic volume controlarrangement for a radio receiver wherein the signal detector functionsas the AVG tube, but its cold gain control electrode maintained positivewith. respect to the cathodes of the controlled amplifier, and whereinan auxiliary bias circuit is provided to supply the normal fixed gridbias for the controlled amplifier until signals of a desired amplitudeare received, and the auxiliary bias circuit embodying as a controlelement thereof a device of uni-directional conductivity which is underthe control of the signal detector, the energization of the signaldetector and the auxiliary bias circuit device being dependent upon atleast two sources of direct current voltage of predetermined magnitude,as well as upon the magnitude of the incoming signal waves.

Another object of the invention is to provide various types of delayedautomatic volume control circuits for radio receivers wherein the delayaction is secured by a special diode circuit whose functioning isdependent upon a predetermined relation between direct current voltages,and whose functioning is interrupted for replacement by an automaticbiasing circuit when signal waves of a predetermined amplitude arereceived.

Other objects of the invention are to improve generally the simplicityand efilciency of automatic gain control networks for radio signalingsystems, and more especially to provide delayed AVC arrangements forradio receivers which are not only reliable and eflicient in operation,but are economically manufactured and assembled in radio receivers.

The novel features which I believe to be characteristic of my inventionare set forth in particularity in the appended claims. The inventionitself, however, both as to its organization and method of operationwill best be understood by reference to the following description takenin connection with the drawings wherein I have indicated several circuitorganizations whereby my present invention may be carried into practicaleffect.

In the drawings:

Fig. 1 shows a circuit diagram of a receiver embodying one form of thepresent invention,

Fig. 2 graphically shows the operation of the invention,

Fig. 3 illustrates a modification of the invention,

Fig. 4 illustrates still another form of the present invention,

Fig. 5 graphically shows the operation of the modification embodied inFig. 4,

Fig. 6 shows a further modification of this present invention,

Fig. '7 illustrates still another modification,

Fig. 8 schematically shows an additional form of the invention,

Fig. 9 shows still another embodiment,

Fig. 10 graphically shows the operation of the embodiment shown in Fig.9.

Referring now to the accompanying drawings, wherein like referencecharacters in the diiierent figures designate similar circuit elements,there is shown in Fig. 1 a superheterodyne receiver of a commercialtype, and which receiver embodies a practical embodiment of the presentinvention which has been successfully operated. The receiver is shown asembodying customary networks preceding the second detector stage, andsuch networks usually comprise a conventional signal collector l, whichmay be of the grounded antenna type; a loop antenna; a pick-up device ofan automobile radio receiver; or even a radio frequency distributionline as commonly employed in hotels and apartment houses.

The signal collector, regardless of its construction, is coupled to thetunable signal input circuit 2, of the converter network whose outputincludes the resonant circuit 3, the latter being fixedly tuned to theoperating I. F. The operating I. F. may be chosen from a range offrequency values depending upon the use to which the receiver is put. Ifthe receiver is of the multi-range type, then the operating I. F. willhave a frequency value depending upon the range in which the receiver isoperated. The mixer,

or first detector-local oscillator, network is shown in conventionalizedmanner, and is to be understood as embodying a tube of the pentagridconverter type. Such tubes are well known to those skilled in the art,and the circuit networks associated with such a tube have been disclosedand claimed by J. C. Smith in application Serial No. 654,421, filedJanuary 31, 1933. For the purposes of the present application, it isbelieved sufficient to point out that the numeral 4 designates apentagrid converter tube, and that the numeral 2 denotes the tunablesignal grid circuit of the tube; the numeral 5 designating the tunablelocal oscillator network, and the numeral 3 denoting the I. F. outputcircuit.

A dotted line is shown between the variable condensers of the circuits 2and 5, and this dotted line represents the uni-control tuning mechanismof the receiver. The details of the pentagrid converter circuit are notshown, but it is to be understood that the signal grid and cathodecircuits of the network are similar to those shown in connection withthe succeeding I. F. amplifier tube 6. Of course, there may be utilizedin place of the composite mixer stage, separate first detector and localoscillator tubes. The I. P. amplifier 6 is shown as of the pentode type,the signal input circuit 1 thereof being magnetically coupled to thecircuit 3 in the customary fashion. The cathode of amplifier 8 isgrounded, and the cathode of the converter tube 4 is to be understood asgrounded.

The plate circuit of the amplifier 6 includes the resonant circuit 8,and the latter is magnetically coupled to a succeeding resonant circuit9 which is disposed between the anode and cathode of the following diodefunctioning as the second detector-AVG control tube. Each of circuits 3,l, 8 and 9 is to be understood as being fixedly tuned to the operatingI. F. Further, if desired the couplings between these circuits may besuch that optimum transmission efficiency is imparted to the networkspreceding the second detector. The numeral l0 designates the doublediode tube which is utilized for performing three functions. One ofthese functions is the demodulation function; a second function is thatof controlling the gain of the preceding tubes 4 and 6; and the thirdfunction is that of providing the normal fixed grid bias for tubes 4 and6'.

The audio component of detected signal energy is impressed upon an audioamplifier l l, and the output of the latter may then be impressed uponan output tube, the final output being reproduced in any desired manner.While in the commercial embodiment of the receiving system shown in Fig.l, the tubes employed are of the so-called metal envelope type. it is tobe clearly understood that the electron discharge tubes may be of theconventional glass envelope type. It will be understood that the grounddesignations shown in connection with tubes 4, 6, H] and II denote thefact that the envelope of the tube is metallic: and performs the dualfunction of housing the electrodes of each tube, and at the same timeshields the electrodes from the remaining portions of the system. Sincethe specific construction of these metal envelope tubes is not a partor" the present invention, they are schematically shown.

The various tubes of the receiving system are energized from the usualpower supply network which is customarily connected to the 60 cycleline. Since such power supply networks are well known at the presenttime, it is only believed necessary to specifically describe thoseportions of the supply network which are of particular utility in thefunctioning of the present invention. Thus, there is connected in thenegative side of the power supply network, and in the outputof the powersupply filter, a resistor 2, a resistor 13, and a resistor i i. Thejunction of resistors 63 and M is grounded, and the posi tive side ofresistor E4 is connected to the positive side of the power supplynetwork through resistors connected in seri The last named pair ofseries resistors are denoted by the numorals i5 and i5, and the junctionof these two resistors is connected by lead l6, for furnishing thepositive voltage for the screen grids of the converter tube 4.

The lead it is by-passed to ground through a condenser i7, and thepositive side of resistor i5 is by-passed to ground through condenser58. The plate circuits of tubes i, E and H are shown energized from thepositive side of resistor l5, and the plate circuit of tube 8! isconnected to the positive side of resistor l5 through resistors i9 andi9 connected in series. The normal fixed grid bias for the signal gridsof tubes i and 5 is provided by the voltage drop across resistor i3.This is so because the cathodes of these tubes are grounded, and thenegative side of resistor 53 is connected to the signal grids of tubes 4and 6 through a path which includes the lead 28, the diode cathode 2!,the diode anode 22, the lead 23, and the AVG lead 24.

It will be observed that the ground side of resistor i3 represents thefixed direct current potential of the cathodes of tubes i and S. Thepositive side of resistor I i, that is the junction of resistors M andi5, is connected to the cathode 25 of the detecting diode of tube l0,through a path which includes lead 26 and resistor 21. The detectiondiode anode 29 is connected to the cathode 25 through the series pathwhich includes the tuned circuit 9, the resistor 38 and resistor 35,resistors 39 and 35 being by-passed for high frequencies by condenserThe direct current voltage component of the rectified signal currentsflowing through resistors 39 and Si is utilized for AVC purposes byconnecting the lead 24 to the junction point 33 through resistor 34.Each of the controlled signal grids is connected to the AVG lead 24 by adirect current connection, and the signal grid of converter tube 4 isconnected to lead 24 through resistor 35.

The audio voltage component of the rectified signal currents isimpressed upon the signal grid of audio tube l l through a path whichincludes the condenser 36 having one side thereof cone ted to thejunction point 33, and its other side connected to ground through themanual volume control resistor 37. The signal input grid of audio tube lis connected to any desired point along resistor 3'! by the adjustabletap 38. The cathode of tube H is connected by lead 39, and

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lead 26, to the positive side of resistor 14, and the voltage dropacross resistor I 4 provides the operating negative grid bias for theaudio amplifier II. In other words the resistor 14 performs the doublefunction of maintaining the detector diode anode 29 at a normal positivepotential with respect to the grounded cathodes of the controlled tubes4 and 6, and at the same time provides the requisite operating negativegrid bias for the sig nal grid of audio amplifier l I, it being notedthat the signal grid of tube H is connected to the ground side ofresistor l4.

By means of this construction the usual by-pass electrolytic condenserused in the grid bias network of the audio amplifier is saved, and itneed not be employed. The grid bias of the audio amplifier followingtube H is derived from across resistor l2. Returning to the double diodetube II], it will be observed that the two diodes have their electrodesshielded from each other by means of the metal shield 40, and the shieldis grounded because of its connection to the metal envelope of tube H1.From a general viewpoint the voltage drop across resistor l3 furnishesthe normal operating grid bias for the signal grids of tubes 4 and 6.After a signal amplitude of a desired intensity has been attained, thisnormal grid bias is replaced by the automatic gain control bias which istapped off from the junction point 33.

Considering now the specific functioning of the delayed AVC system shownin Fig. 1, attention is directed to Fig. 2 which graphically shows themanner in which the fixed bias diode is replaced by the AVG diode uponthe reception of a signal of a predetermined amplitude. From Fig. 2 itwill be noted that up to a predetermined amplitude of signal the diode 2l--22 is active, or conductive, thereby permitting the signal grids oftubes 4 and 6 to be at substantially the direct current potential of thecathode 2|. In other words, during the entire period that the diode2I-22 is active, the voltage drop across resistor 13 furnishes the solenegative bias for the signal grids of the controlled tubes; and this isso be cause the cathodes of these tubes are connected back to the groundpoint of resistor I3. Also, during this period the anode 22 is at apotential with respect to the cathode 2| which is equal to the voltagedrop across resistors 13 and 14.

Furthermore, during the period when the negative grid bias for tubes 4and B is derived across resistor 13, the detector diode anode 29, beingconnected to the positive side of resistor I4, is at a positive directcurrent potential with respect to the grounded cathodes of controlledtubes 4 and 5. As soon as signals are received Whose amplitude exceedsthe voltage developed across resistors i3 and I4, then the diode 2|22 isrendered inactive, or non-conductive. This occurs by virtue of the factthat the anode 22 is connected through lead 23 to the junction point 33.The resistor 34 functions as a filter resistor, and it is also pointedout that resistor 21 and condenser 21 function as a filter network tobypass low frequency hum. In Fig. 2 the upwardly ascending curve showsthe manner in which the negative bias on the signal grids of controlledtubes 4 and 6 increases as the signal strength increases when the diode2l22 has been rendered inactive.

At the point when the received signal amplitude exceeds the voltageacross resistors l3 and I4 the negative bias applied through lead 24 isequal to the voltage developed by rectification in the circuit of diode2529 minus the positive voltage developed across resistor l4. It will beseen that when the diode 2 I-22 has been rendered inactive, then thebiasing path through resistor I3 is broken. It will now be appreciatedthat without utilizing the usual self-bias networks in the circuits oftubes 4 and 6, a normal fixed grid bias has been provided; andadditionally, the AVG action has been delayed until a signal of apredetermined amplitude is received. Upon the AVG action be comingoperative, the fixed bias device becomes inoperative. It is alsoimportant to note that during normal operation of the receiver, the coldelectrode of diode 2925, the signal detector, is maintained positivewith respect to the cathodes or the controlled tubes.

Merely by way of illustration, and in no way limiting, the followingspecific examples are given of circuit constants which may be employedin conjunction with the circuit shown in Fig. -1. Assuming that thetubes employed in the converter and I. F. amplifier stages utilize anormal fixed bias of -3 volts, and that the tube 1 l is of a type whichcan utilize a grid bias of 1.5 volts, the following circuit constantsmay be utilized:

The audio transmission leads between junction point 33 and the signalgrid of tube II are shown shielded. The dotted line designation 4!, andthe dotted line 42, are to be understood as comprising grounded shieldsfor their respective leads. Again, it is emphasized that the presentinvention is applicable to additional circuit arrangements.

For example, the fixed bias diode 2l-22 need not necessarily be embodiedin the same tube as the demodulator diode 29-25. This diode 2l-22 may beembodied in any of the other tubes, since its function is to provide thefixed initial bias during normal reception.

In Fig. 3, there is shown a modification of the invention wherein thefixed bias diode is disposed in the tube envelope of the audio amplifierH.

The circuit elements of the embodiment shown in Fig. 3 are substantiallythe same as those shown in Fig. 1; however, many of the circuit elementshave been omitted which are duplicates of those shown in Fig. 1. It willbe observed that the power supply network includes the ground connectionat the junction of resistors l3 and I4, and that the cathode 25 of thedemodulator tube i is connected through its lead 26 to the positive sideof resistor E4. The fixed bias diode anode 22 is connected through itsleads 23 and 24 to the signal grids of tubes 4 and 6. The cathode of theaudio tube I l is connected to the junction of resistors l2 and I3,instead of to the positive side of resistor M as in Fig. 1.

This change must be made because the signal grid of tube 1 I must bemaintained at a negative potential with respect to the cathode of thetube. For this reason, the signal grid of tube II is connected throughresistor 31 and tap 31 to a point on resistor l2 which is at a negativevoltago with respect to the point thereon to which the cathode of tubeII is connected, and the value of this voltage is equal to that desiredfor normal operation of the audio tube. The tube II may be of the typewell known to the art as a duplexdiode triode, wherein the diode anodesare strapped together to provide the single anode 22'. In thismodification, then, the fixed bias diode is in another tube envelope,and yet all the functions noted in connection with Fig. 1 are stillobtained.

The arrangement shown in Fig. 4 in general embodies the circuit elementsshown in Fig. 1. Only those portions of the circuit are shown which areessential to a clear understanding of the differences in the embodiment.The delayed AVC system shown in Fig. 4 is usually employed in commercialpractice in connection with a multi-range superheterodyne receiver whichuses separate first detector and local oscillator tubes, and a stage ofradio frequency amplification ahead of the first detector. Further, thereceiver is provided with a tuning indicator 50 of any conventional andwell known type. The purpose of the tuning indicator is to facilitatethe tuning manipulation, the latter being rendered somewhat moredifiicult by the inclusion of the AVG system.

To provide the actuating negative bias for the tuning indicator device50, the junction of the detector diode load resistors 30 and 3| isconnected to ground through a path which includes resistors 5i and 52,the junction 53 of the series resistors being by-passed to groundthrough condenser 54. The negative bias for the tuning indicator 50 isderived from the junction point 53,and it will be observed that the lead55 over which this partial bias is transmitted, is shielded. The AVCbias is transmitted through the lead 24, the latter being connected inthis case to the anode side of resistor 30 through a resistor 34', andthe audio component of rectified signal currents is transmitted to theaudio tube II by the audio connection to the junction point of resistors30 and 3 I, the latter connection being similar to that shown in Fig. 1.

Otherwise the circuit arrangement shown in,

Fig. 4 is similar to that shown in Fig. 1. The anode 22 of the fixedbias diode is connected through lead 23 to the AVG lead 24, and thecathode 2! is connected through lead to the negative side of resistorI3. The demodulator diode cathode 25 is connected through lead 26 to thepositive side of resistor I4. Of course, by proper proportioning of theresistors 5| and 52, any partial negative bias, or any fraction desired,may be supplied to the tuning indicator 50. The input circuit 9 of thedemodulator diode has its coil arranged to buck out hum frequencies; thecircuit constants may be changed to suit the purposes of the presentreceiver. For example, resistors I3 and I4 may now have values of 28 andohms respectively; resistor 36 may have a value of 2.2 inegohms;resistors 5! and 52 may have values of 2.2 megohms and 880,000 ohms,respectively.

The audio tube II is here shown as employing a self-bias resistor 56,by-passed in the usual manner by condenser 51. This self-bias network isemployed where the tube II is of a type requiring a higher negative biasthan that developed across resistor I4. For example, a tube of the typeshown at I! may utilize a bias of 9 volts, and therefore, could notutilize the lesser negative voltage developed across resistor I4. Themanual volume control connection between the diode demodulator and audiotube II has been simply shown, but it may be of the tone compensatedtype if desired. It is to be noted that unlike the arrangement shown inFig. 1, there is no need to utilize in this modification the filternetwork 27-21.

In Fig. 5 there is shown the operation of the system disclosed in Fig.4; the two curves shown in this figure illustrating the difference invariation between the negative bias supplied to the tuning indicator,and that supplied to the controlled tubes. The curve A is the same asthe curves shown in Fig. 2. The curve B shows the variation in negativebias at junction point 53 in Fig. 4. It will be seen that the negativebias for the tuning indicator develops as soon as signal currents arerectified across the demodulator load resistors. This is explained bythe fact that the bias transmitted to the tuning indicator is notaffected by the fixed bias diode network.

It is also to be noted in these difierent circuit arrangements embodyingthe present invention that a certain degree of delayed detection issecured by virtue of the voltage drop across resistor 3|. When it isrealized that this resistor is disposed in the path of the fixed biasdiode, it will be seen that the diode demodulator 29-45 will not rectifyuntil received signals have attained at least that amplitude which willexceed the voltage developed across resistor 3!. This is best seen byreference to Fig. 1 wherein it will be seen that the resistor 3| is inseries between the electrodes 2I and 22 with the voltage drops acrossresistors I3 and i4. Such delayed detection reduces the noisereproduction to a great extent.

In each of Figs. 1, 3 and 4 the grids of the controlled tubes are shownconnected to the same control bias point of the AVG network. Such amanner of connecting the circuits is not essential since the controlbias may be applied to the successive controlled tubes in graduallydiminishing intensity. In Fig. 6 such a modification is shown.

The circuit arrangement of Fig. 6 shows the various stages thereof inconventionalized form. The detector is shown as preceded by cascadedradio frequency amplifiers, and followed by an audio amplifier. It is tobe clearly understood that these radio frequency tubes may be thevarious stages of a superheterodyne receiver which precede the seconddetector; as, for example, shown in Fig. 1. In any case, the symbol D1designates the demodulator diode whose cathode includes the resistor I2having one side thereof grounded. The cathode of the audio amplifier isconnected to the cathode side of resistor l2, as disclosed in connectionwith Fig. 1. The direct current voltage component of the rectifiedsignal currents is utilized for AVC action by transmitting it throughresistor II to the AVG leads. The fixed bias diode D2 has its anodeconnected to the resistor 'II, and its cathode is connected to groundthrough the resistor ID.

A condenser M is connected in shunt witli the diode D2, and the resistorI5 is connected in shunt with the diode D2 and condenser M. The negativelead of the power supply network is connected to the cathode side ofresistor I5, and the voltage drop across resistor IIJ furnishes thenormal grid bias for the radio frequency tubes preceding thedemodulator. To secure the benefits of graduated control on the cascadedamplifiers, the grid of the second controlled tube is connected to thepoint '55 on resistor 15, and the grid of the tube preceding thedetector is connected to the point It Will be noted that the cathodes ofeach of the controlled tubes is grounded, and that the AVG leads to thegrids each include filter resistors.

The arrangements shown in Fig. 6 function in a manner similar to thatdescribed in connection with Fig. l. The diode D1 not only furnishes theAVG bias, but also supplies the audio signal for the succeeding audioamplifier. The resistor 15 is a bleeder resistor for the purpose ofapply ing AVC bias to the successive grids of gradually diminishingnegative values. The bleeder 15 in turn is connected to a point in thesupply circuit which is below ground potential by the voltage dropacross resistor 10. When the diode D2 is conductive, then the voltagedrop across resistor l0 furnishes the normal fixed bias for the variousgrids of the controlled tubes.

The resistor H is made sufiiciently large not to interfere with thedetection action occurring in the circuit including diode D1. When nosignals are received, the .anode of diode D2 is positive with respect toits cathode by the potential equal to the drops across resistors "Hi and12. This causes the diode D2 to be conductive, and the anode side of thediode is at substantially the same direct current potential as thecathode side thereof. When signals are received, which are of anamplitude suiiicient to just overcome the aforementioned voltage dropsacross resistors 70 and 12, then the diode D2 becomes inactive, and theAVG action is the biasing element. Here, again, it is to be noted thatthere will be a slight biasing voltage developed across resistor T3,assuming that the resistor 13 causes an appreciable voltage drop, withthe result that there will be a suppressor action for extremely weaksignals.

In Fig. '7 is shown a modification of the arrangement in Fig. 6 whereinthe diode D2 is non conductive for low signal intensities. It becomesconductive when signals are received which are strong enough to overcomethe potential difference between points y and at. It will be observedthat this is the reverse of the situation shown in Fig. 6, and issecured by connecting the diode D2 in series between the bleeder l5 andresistors H and 73. It is believed that the operation and functioning ofthis modification will be obvious from the description given inconnection with Fig. 6.

In both Figs. 6 and '7 the diodes D1 and D2 may be disposed in a singletube envelope. Furthermore, at least one of the diodes may be disposedin the envelope of another tube, as shown in connection with Fig. 3.While Figs. 6 and "I disclose the application of different AVC voltagesto the controlled tubes, it is within the scope of the present inventionto more generally provide different initial voltages and different delayaction to the controlled tubes.

In Fig. 8 there is shown a circuit diagram of a receiving systemembodying such a circuit wherein the controlled tubes are givendifferent delays and different initial voltages by means of a pluralityof auxiliary diodes. Each of the controlled tubes 80, 8! and 82 isprovided with self-bias networks 83, 34 and 85. The diode D1 is thedemodulator, and it is to be understood that the controlled tubespreceding it may be the customary superheterodyne receiver networks, orthey may be amplifiers which are tunable through a common signalfrequency range.

The audio voltage component developed across resistor 86 is fed to thesucceeding audio amplifier, and the latter includes in its cathodecircuit the resistor Bl which develops voltage 61.

An intermediate point on resistor 86 is connected to three parallelbleeder resistors, the grid of tube 83 being connected to anintermediate point on the first bleeder resistor 88; the grid of tubebeing connected to an intermediate point on the second bleeder resistor,and the grid of tube 82 being connected to an intermediate point on thethird bleeder resistor 89. In order to explain the operation of thismodification, the biasing actions in connection with tube Bl will beconsidered. The initial bias on this tube is equal to minus (ea-tea).The delay bias on tube at is equal to (e1+e2). The fraction of the AVGvoltage applied to tube BI is equal to It is believed that the biasingactions with respect to tubes 89 and 82 will be clear from theexplanation in connection with tube 8|. Furthermore, it is pointed outthat the point a in the circuit of diode D1 need not be connected to thecathode of the succeeding audio amplifier, but may be tied to anyavailable potential. The numerals 98, 9| and 92 designate the auxiliarydiodes operatively associated with the parallel bleeder resistors.

In Fig. 9 there is shown a further modification wherein the presentinvention is applied to a circuit which will permit the control voltageon a given tube to be varied within predetermined limits as may bedesirable to avoid undue distortion from one stage. The controlled tubeI00 utilizes in its cathode circuit a self-bias network, and the voltagedeveloped across the self-bias resistor is denoted by the symbol @4. Thediode demodulator D1 has an intermediate point on its load resistor IUIconnected through resistor :02 to the anode of an auxiliary diode D2.

The cathode of the latter diode is connected to ground through aresistor which develops a voltage 66, and the cathode of diode D1 isconnected to ground through a resistor which develops the voltage 65.The diode D2 is shunted by resistor H13, and an intermediate pointthereon is connected to the cathode of a second auxiliary diode D3. Theanode of diode D3 is connected to resistor Hi3 through a resistor whichdevelops a voltage 67. The cathode of diode D3 is connected to the gridof the controlled tube through resistor I 04.

The curve in Fig. 10 illustrates the biasing actions in Fig. 9. Thevoltage on the control grid of tube I00 varies as shown in Fig. 10, avariable negative bias being applied to the grid between negative biasvalues of constant intensity 6a. and 6b. The values of these latterlimiting constant negative bias values is designated in Fig. 10 in termsof the various voltages shown in Fig. 9. From Fig. 10 it will beobserved that the AVG action applies a variable negative bias to thecontrolled tube between predetermined limits.

While I have indicated and described several circuit arrangements forcarrying my invention into effect, it will be apparent to one skilled inthe art that my invention is not limited to the particular systemsdescribed herein, but that many additional modifications may be madewithout departing from the scope of my invention as set forth in theappended claims.

What I claim is:

1. In a radio receiver provided with a signal demodulator including anelectron emission element and a cold output electrode, a load impedanceconnected in circuit with said element and electrode, a signaltransmission tube in said receiver which includes at least a signalgrid, a cathode and an anode, the anode being coupled to the demodulatoroutput electrode for the impression of signals upon the demodulator,said tube cathode being maintained at a predetermined fixed directcurrent potential, an automatic gain control connection between thecontrol grid of said tube and the point on said load impedance adjacentthe demodulator output electrode, a normal grid bias path connectedbetween said control grid and a potential point which is at the samepotential as the cathode of said signal transmission tube, said normalbiasing path including in series a diode and a source of negativepotential of a predetermined value, a second source of negative directcurrent potential having its negative terminal connected to said lastnamed potential point and its positive terminal connected to theemission element side of said demodulator load impedance.

2. In a receiver as defined in claim 1, said diode having its cathodeconnected to the negative terminal of said first source of negativepotential, and said demodulator comprising a diode.

3. In a receiver as defined in claim 1, said first and second potentialsources comprising bleeder resistors connected in the power supplynetwork of the receiver.

4. In a radio receiver provided with a plurality of cascaded signaltransmission tubes, each tube including a cathode at ground potential, ademodulator diode including a load resistor between its anode andcathode, means connected between ground and the cathode side of saidload resistor for maintaining the demodulator anode normally positivewith respect to said grounded cathodes, means for applying a normalnegative bias to the signal grids of said cascaded tubes, said lastmeans including a diode having its anode connected to said signal grids,and its cathode connected to ground through a source of negative directcurrent potential, and a gain control connection between the said signalgrids and a point on said load resistor which is at a negative directcurrent potential with respect to the demodulator cathode when signalsabove a predetermined amplitude are received.

5. In a receiver as defined in claim 4, an audio amplifier followingsaid demodulator, and connections between the input electrodes of saidaudio amplifier and said first named means for maintaining the signalgrid of the audio amplifier at a negative direct current potential withrespect to the cathode thereof.

6. In a receiver as defined in claim 4, the electrodes of both saiddiodes being disposed within a common tube envelope.

'7. In a receiver as defined in claim 1, additional means for impressingthe direct current potential developed across said demodulator loadresistor upon the signal grids in progressively decreasing manner.

8. In a radio receiver of the type utilizing a rectified alternatingcurrent supply network for energizing the tubes of the receiver, andsaid receiver being of a type including a plurality of cascaded signaltransmission tubes followed by a demodulator tube, the improvement whichcomprises a connection between the cathode of the demodulator and apoint in said power supply network which is at a positive potential withrespect to ground, a load resistor connected between the cold electrodeof the demodulator and its cathode, the cathodes of said transmissiontubes being maintained at ground potential whereby the demodulator coldelectrode is normally positive with respect to ground, a signal gridbiasing connection between the signal grids of said cascaded tubes and apoint on said supply network which is negative with respect to ground,and a control connection between the cold electrode side of saiddemodulator load resistor and said grid biasing connection for renderingthe latter inoperative to bias the signal grids when signals above apredetermined amplitude are received.

9. In combination with a wave transmission tube having at least acathode, a wave input electrode and an output electrode, a wavedemodulator diode having its electrodes coupled to the output electrodeof said tube, a load resistor connected between the anode and cathode ofsaid diode, means for normally maintaining the diode anode at a positivedirect current potential with respect to the cathode of the wavetransmission tube, additional means including a non-rectifying device ofuni-directional conductivity for maintaining the wave input electrode ofsaid tube at a negative direct current potential with respect to itscathode, and means responsive to a predetermined signal amplitudeincrease for rendering said additional means inoperative, and biasingsaid input electrode in direct proportion to the wave amplitudeincrease.

10. In a receiver as defined in claim 8, an audio amplifier followingsaid demodulator tube, an audio connection between the input electrodeof said audio amplifier and an intermediate point on said demodulatorload resistor, and a bleeder resistor connected between said last namedintermediate point and ground, there being a connection from apredetermined intermediate point on said bleeder resistor to a tuningindicator.

11. In a receiver as defined in claim 9, said additional means includinga diode connected in series with a source of negative direct currentpotential, and said last named diode and the demodulator diode havingtheir electrodes disposed in a common tube envelope.

12. In a receiver as defined in claim 9, an audio amplifier followingsaid demodulator diode, and said additional means being connected to theinput electrodes of said audio amplifier for maintaining the signalinput electrode of the latter at a desired negative potential withrespect to the cathode thereof.

13. In a receiver as defined in claim 9, an additional diode operativelyassociated with the said wave input electrode and said last named meansfor maintaining the said wave input electrode at a predeterminedconstant negative potential with respect to its cathode upon impressionon said wave transmission tube of waves above a predetermined amplitude.

14. In a receiver as defined in claim 9, said last means including adiode, and said last diode also being included in said additional means.

15. In a radio receiver provided with a plurality of cascaded signaltransmission tubes, each tube being provided with at least a signalcontrol grid, a cathode, and an anode, a demodulator diode following thelast of the cascaded tubes, said diode including a load resistor fordeveloping direct current and audio frequency components from receivedsignal Waves, gain control connections from the signal grids of thecascaded tubes to a point on the demodulator ioad resistor which is at anegative direct cur rent potential with respect to the demodulatorcathode potential when signals above a predetermined amplitude arereceived, each of said gain control connections additionally beingconnected to a point of negative direct current potential with respectto ground, and the cathodes of said cascaded tubes being connected toground, additional means for maintaining the demodulator anode at apredetermined positive potential with respect to ground, each of saidgain control connections additionally including a diode constructed andarranged to have its conductivity dependent upon the magnitude of thedirect current potential of said negative point on the demodulator loadresistor.

16. In combination with a signal transmission tube and a signalrectifier having its rectifying electrodes coupled to the said tube, aload resistor connected with said electrodes and across which a directcurrent voltage is developed by rectified signal currents, a biasingcircuit for the input electrodes of said transmission tube comprising adirect current voltage source connected between said input electrodeswith a device functioning to close the biasing circuit, and means forapplying at least a portion of said developed voltage to the said inputelectrodes as a gain control bias, and additional means for applying atleast a portion of said developed voltage to said device in a sense tocause the biasing circuit to become ineffective when the signalintensity assumes a predetermined value.

POUL F. G. HOLST:

